In a sewing machine of the prior art wherein a work may be fed on a work supporting surface through cooperation between a feed dog and a presser foot, provision is made of a pressure imparting spring designed for applying a downward pressure to the presser foot and of manual adjusting means for adjusting the pressure of the spring. In advance of the stitching operation, the downward pressure is set or adjusted in dependence upon the quality and/or thickness of the work so as to effect the stitching operation under such preset condition.
In present-day, however, over edge sewing machines have a tendency to be operated at higher and higher speed, and high speed sewing machines with the rotational speed of the main shaft approaching to 10,000 r.p.m. are now offered to the market. With such high speed machines, various adverse effects may be caused to exist when the sewing is carried out under the same downward pressure conditions as preset at the start of stitching operation. That is to say, when the downward pressure has been set to a higher value suited for high speed operation, the downward pressure thus set may act strongly on the work to mar the same during the starting or on the occasion of low speed operation of the machine in the course of stitching operation. Conversely, when the downward pressure has been set to a lower value suited for low speed operation, should the sewing machine be operated at some higher speed during stitching operation, the presser foot may float on account of high speed operation of the feed dog, thus interfering with the smooth feed of the work.
In order to provide the machine with a solution to such problems, "Speed responsive presser device for sewing machines" has been proposed in the art such as shown in the specification and drawings of the U.S. Pat. No. 2,541,888.
However, in this speed responsive presser device, use is made of a mechanical governor on the main shaft for imparting to the presser foot a downward pressure corresponding to the speed of the main shaft of the sewing machine. Such governor, however, is not suitable for use on the present-day high speed sewing machines with the rotational speed of the main shaft approaching to 10,000 r.p.m. and thus incapable of producing a downward pressure accurately responsive to the main shaft speed. Moreover, such mechanical governor has much to be desired as to durability, noise generation and so forth.
"Fluid pressure actuated mechanism for sewing machines" is also known as disclosed in the specification and drawings of the U.S. Pat. No. 2,750,908.
In this fluid actuated mechanism, the air from an air pump mounted to the output shaft of the power transmitter having a clutch and brake is conducted to an air cylinder mounted on an arm of the sewing machine to impart the downward pressure to the presser foot through such air cylinder. In consideration that the air has compressibility markedly higher than that of liquids, it is by no means feasible to impart to the presser foot a downward pressure accurately responsive to the high speed of the main shaft approaching to 10,000 r.p.m. as encountered in present-day sewing machines.
Further, said air pump is mounted on the output shaft of the power transmitter and a long and thick flexible hose must be used to interconnect said air pump and the air cylinder mounted on the arm of the sewing machine. Hence, the number of component parts may be increased, thus not only resulting in increased manufacture costs but detracting from the overall appearance of the sewing machine.
In view of demands of the times on the enhanced operational speed of the sewing machine and resulting improvement in operational efficiency, the present inventors have conducted a variety of researches and experiments, and devised a sewing machine free of the above deficiencies of the prior art.